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Diapsida : Archosauromorpha : Archosauriformes
The ancestral archosaurs probably originated some 250 million years or so ago, in the late Permian Period. Their descendants (such as the dinosaurs) dominated the realm of the terrestrial vertebrates for a majority of the Mesozoic Era. Today, only the birds and crocodilians exist to provide a glimpse into the past glory of archosaurs.
True archosaurs (the Archosauria proper), including the major stem groups Pseudosuchia (the crocodilians and their relatives) and the Ornithosuchia (the birds and their relatives) evolved yet more new features. The fifth toe in the foot, homologous with your "pinky toe" was reduced in size. In earlier vertebrates, the palate (roof of the mouth) bore at least one row of accessory teeth, but the archosaurs appear to have lost this feature, as did many other lineages of tetrapods. Finally, the two main lineages of archosaurs each have a specialized ankle joint that changed differently in each group, as shown below.
The Pseudosuchia have what is called a crurotarsal, or "crocodile-normal" ankle. This is a very flexible arrangement in which the astragalus (medial proximal ankle bone) bears a peg that fits into a socket in the calcaneum (lateral proximal ankle bone). Ankle rotation then occurs between these two bones, permitting both a somewhat erect stance (like the crocodilian "high walk") where the hindlimb is held closer to the midline of the body, and a more sprawling stance like that of earlier tetrapods. The result of this is that the pseudosuchians can move in two different ways; walking with an erect or sprawling posture.
The Ornithosuchia have a mesotarsal ankle, which is a simple hinge joint between the lower leg and astragalus and calcaneum, and the distal ankle bones. This restricts the posture to a more erect orientation, so the gait can be called parasagittal the limbs move parallel to the vertebral column, and are held relatively vertical. Birds and most mammals have this parasagittal gait; birds inherited it from their dinosaurian ancestors, while mammals evolved it independently. The advantage of a parasagittal gait might be that it improves maneuverability/agility. A concordant disadvantage would be that it reduces stability. It's easier to tip over an erect cow than it is to tip over a more sprawling crocodile of similar size; the crocodile has a wider base of support, and thus can be said to be more stable. However, the relationship between stance and stability is still not completely understood, so it's difficult to say exactly what advantages an erect stance has. Another proposed advantage of an erect stance and parasagittal gait was that it was more efficient, but this has not held up to experimental analysis erect animals move about as efficiently as similarly-sized sprawling ones.
It is difficult to say what sorts of soft tissues extinct archosaurs really had, but the fact that both crocodilians and birds have a four-chambered heart lends support to the notion that this is a trait inherited from a common ancestor (and another convergence with mammals). Birds and crocodiles also share expanded pneumatic sinuses in their skulls and elsewhere in bird skeletons which can also be seen in fossil archosaurs, such as the hadrosaurs, so the expansion of these craniofacial air sinuses is another trait shared by archosaurs. The sinuses appear to serve as skull-remodeling agents, reshaping the bones of the skull in response to stresses and other influences.
The fossil record
The Upper Triassic (about 225 million years ago) witnessed the appearance of several new groups of archosaurs, some of which have living descendants today. Most other archosaurs and archosauromorphs did not make it into the Jurassic, but the crocodilian lineage did, as did the dinosaurs and the tenacious champsosaurs, which all appeared at about the same time in the Upper Triassic. Close relatives of the dinosaurs such as the pterosaurs and the possibly dinosaurian Eoraptor and Herrerasaurus also show up in the Upper Triassic.
The survivors of the Upper Triassic-Lower Jurassic extinctions went on to dominate the rest of the Mesozoic Era, but not without hitches. The Upper Cretaceous Period (around 70 million years ago) was a time of global change and "ecological reshuffling," when many taxa did not make the cut. Pterosaurs, many dinosaurs (including some major bird groups), and some crocodilians vanished around this time.
As the Cenozoic Era dawned 65 million years ago, things were different crocodiles and champsosaurs were still doing well, but the dinosaurs had taken a one-two punch, and only the neornithine birds persevered to continue the massive diversification that began in the Cretaceous Period. Today we are left with only the crocodilians and the birds as extant (living) archosaurs.
Something to keep in mind
Text by John R. Hutchison, 10/1995, with additions by Brian R. Speer and John, 1/1998; additional text by Matt Wedel, 5/2007.
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